Geometry is always
clipped against the boundaries of a six-plane frustum in x, y, and z. glClipPlane
allows the specification of additional planes, not necessarily perpendicular
to the x, y, or z axis, against which all geometry is clipped. To determine
the maximum number of additional clipping planes, call glGetIntegerv with
argument GL_MAX_CLIP_PLANES. All implementations support at least six such
clipping planes. Because the resulting clipping region is the intersection
of the defined half-spaces, it is always convex.

glClipPlane specifies a
half-space using a four-component plane equation. When glClipPlane is called,
equation is transformed by the inverse of the modelview matrix and stored
in the resulting eye coordinates. Subsequent changes to the modelview matrix
have no effect on the stored plane-equation components. If the dot product
of the eye coordinates of a vertex with the stored plane equation components
is positive or zero, the vertex is in with respect to that clipping plane.
Otherwise, it is out.

To enable and disable clipping planes, call glEnable
and glDisable with the argument GL_CLIP_PLANEi, where i is the plane
number.

All clipping planes are initially defined as (0, 0, 0, 0) in eye
coordinates and are disabled.